When I got one of those big thick tax assesment forms the other month, I found after some investigation that I should never have received it.
If you pay by Quellensteuer (tax at source), and earn less then 120K CHF a year, you don't need to fill out tax returns unless you really, really want to

(in Basel-Stadt at least)!
I was advised by serveral people that the only reason to fill it out is if you want something like extra pension payments taken into account to reduce the tax you pay, or if you think you've overpaid. A quick call to the tax dept got the tax return cancelled for me.
I have been warned that if in the future I do start filling these things out they may demand I do a couple of years worth retrospectively, but only one person (there were 30-odd people in room when we had this discussion, most of them expats who've been here from weeks to years) had actually got hit by that.
Hope this helps